The subject matter of the present invention relates to a method and apparatus for passively and continuously monitoring the status of seawater advance toward water acquifers near coastal cities by placing sensors, such as a particular type of electrode cable system, in the ground near observation and injection wells, transmitting acquired data back to centralized processing facilities, and, responsive thereto, subsequently mitigating the advance of the seawater into potable water acquifers which are situated near the coastal cities.
Coastal cities rely on groundwater from subsurface aquifers to meet all or part of their municipal water needs. In cases of historical overdraft, water is withdrawn from the subsurface aquifers at a rate exceeding the rate of natural aquifer recharge. As illustrated in FIG. 1, such overdraft results in a lowering of the water table in the aquifers and is accompanied by possible intrusion of seawater into the aquifer. The breakthrough of seawater at wells supplying the drinking water has severe long term consequences on municipal potable water deliverability. For example, Los Angeles (LA) experienced such overdraft in the first half of the twentieth century (the 1900s). As a result, LA subsequently created the Water Replenishment District (WRD) agency to define and enforce reduced aquifer pumping rates and mitigate the effects of seawater intrusion.
Various means exist to mitigate seawater advance. One of these methods is to recharge the aquifers from surface spreading grounds. Another method involves injecting inert gas or fresh water into the ground using special injection wells, as illustrated in FIG. 2. The water injection method consists of injecting fresh water into the aquifer, forming a fresh water xe2x80x98moundxe2x80x99 in the local area around the well, to create a zone with pressure above the pressure in the seawater, thereby mitigating the landward advance of the seawater. As an example, since the 1950s, Los Angeles has constructed approximately 250 seawater barrier injection wells of this type thereby resulting in three lines of injectors. These three different lines of injectors are illustrated in FIG. 3, where the position of the water injection wells is shown by the adjacently connected circles.
In addition to the wells constructed for water injection (i.e., injection wells) as shown in FIGS. 2 and 3, a number of observation or monitoring wells (i.e., observation wells) are typically constructed in the vicinity of the injection wells. These observation wells are used to periodically measure the pressure (i.e., the hydraulic head) of the aquifer in the neighborhood of the injection wells, and for occasional sampling of water chloride (i.e., salinity) levels. This gives information about how efficiently the injection wells are limiting the seawater advance. In Los Angeles, for example, over 700 observation wells have been constructed along the three lines of injection wells (shown in FIG. 3) in order to monitor the position of the seawater wedge. In a typical municipal setting with seawater advance into the aquifers, the water authorities sample the chloride concentrations at three positions (top, middle, and bottom) of each water sand unit, as illustrated in FIG. 7. This provides information about how efficiently the injection wells are limiting the seawater advance in each sand unit.
However, a need exists to passively and continuously monitor, in the observation wells, the status of the seawater wedge and the resultant seawater advance toward coastal city water aquifers as well as the status of the injected fresh water in the injection wells.
Depending on the application, various types of in-situ sensors have been employed in the oil industry for general reservoir monitoring. See the following xe2x80x9cfirst referencexe2x80x9d which discloses general reservoir monitoring: Babour, K. A., Belani and J. Pilla, xe2x80x98Method and Apparatus for Surveying and Monitoring a Reservoir Penetrated by a Well Including Fixing Electrodes Hydraulically Isolated within a Wellxe2x80x99, U.S. Pat. No. 5,642,051, the disclosure of which is incorporated by reference into the specification of this application. In addition, such sensors have been proposed for leak detection, soil heating and temperature mapping. See the following three xe2x80x9csecond set of referencesxe2x80x9d which disclose leak detection, soil heating, and temperature mapping: (1) Berryman, James G., Daily, William D., xe2x80x98Optimal joule heating of the subsurface, U.S. Pat. No. 5,325,918, the disclosure of which is incorporated by reference into the specification of this application, (2) Daily, William D., Laine, Daren L., Laine, Edwin F., xe2x80x98Methods for Detecting and Locating Leaks in Containment Facilities using Electrical Potential Data and Electrical Resistance Tomographic Imaging Techniquesxe2x80x99, U.S. Pat. No. 5,661,406, the disclosure of which is incorporated by reference into the specification of this application, and (3) Ramirez, Abelardo L.; Dwayne A.; Daily, William D., xe2x80x98Using Electrical Resistance Tomography to Map Subsurface Temperaturesxe2x80x99, U.S. Pat. No. 5,346,307, the disclosure of which is incorporated by reference into the specification of this application.
Consequently, in connection with the aforementioned need to passively and continuously monitor, in the injection wells and the observation wells, the status of the seawater wedge and the resultant seawater advance in addition to the status of the injected fresh water, there is a further need to utilize xe2x80x98special sensorsxe2x80x99 in the injection wells and in the observation wells to perform the step of monitoring the seawater advance and the status of the injected fresh water. These xe2x80x98special sensorsxe2x80x99 can be the sensors disclosed above in connection with the xe2x80x9cfirst referencexe2x80x9d or in connection with the xe2x80x9csecond set of referencesxe2x80x9d. Alternatively, these xe2x80x98special sensorsxe2x80x99 can be new sensors which are adapted for the above stated purpose of monitoring the seawater advance and the status of the injected fresh water.
Accordingly, it is a primary object of the present invention to passively and continuously monitor, in the observation wells, the status of the seawater wedge and the resultant seawater advance toward fresh water aquifers located near coastal cities in addition to the status of any fresh water injected into the injection wells.
Accordingly, it is a primary aspect of the present invention to permanently install sensors in the ground in or around an observation well located near the injection wells in order to passively and continuously monitor the status of the seawater advance toward fresh water aquifers near coastal cities in addition to the status of the fresh water injected into the injection wells (hereinafter, the xe2x80x98monitoring stepxe2x80x99).
It is a further aspect of the present invention to implement the aforementioned xe2x80x98monitoring stepxe2x80x99 by utilizing sensors which have been used in the oil industry, such as the sensors used for general reservoir monitoring, and/or the sensors used for leak detection, soil heating, and temperature mapping.
It is a further aspect of the present invention to implement the aforementioned xe2x80x98monitoring stepxe2x80x99 by utilizing sensors that are specially designed for use during the steps of monitoring the seawater advance toward fresh water aquifers near coastal cities and monitoring the status of fresh water injected into the injection wells.
It is a further aspect of the present invention to use oilfield related techniques/methods [that are currently being used in the oil industry to detect and record the existance of underground deposits of hydrocarbon (such as oil)] for the purpose of: (1) detecting the advance of seawater toward fresh water aquifers near coastal cities, and (2) detecting the pressure in the xe2x80x98moundxe2x80x99 of fresh water that has been injected into injection wells, the purpose of which is to create a zone of pressure above the pressure in the seawater for mitigating (i.e., slowing) the landward advance of the seawater toward the fresh water aquifers that are situated near the coastal cities.
It is a further aspect of the present invention to install a resistivity array around a casing of an observation well, or inside the casing of the observation well, which is located near an injection well, for the purpose of measuring the conductivity and resistivity of the earth formation within or around the observation well, the conductivity and resistivity values being representative of the presence or absence of either seawater or fresh water within or near the observation well, the conductivity and resistivity values being further representative of the location of a seawater/fresh water boundary within or near the observation well, the boundary being further representative of the advance of the seawater toward fresh water aquifers near coastal cities.
It is a further aspect of the present invention to install a resistivity array within or around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, the resistivity array having a top subarray portion, a middle subarray portion, and a bottom subarray portion for measuring and determining the conductivity and resistivity of a top part, a middle part, and a bottom part of the water sand unit thereby determining the location of said seawater/fresh water boundary.
It is a further aspect of the present invention to install a resistivity array around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, the resistivity array including a plurality of subarrays located adjacent the top part and the middle part and the bottom part of the water sand unit, each subarray of the resistivity array including a plurality of electrodes, a first pair of the electrodes generating and receiving current, a second pair of the electrodes used to measure the resulting potential difference, the current and the potential difference being used to calculate the resistivity of the water sand unit, the resistivity being indicative of the presence or absence of seawater in the water sand unit near the observation well.
It is a further aspect of the present invention to install a resistivity array around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, wherein the electrodes which comprise each subarray of the resistivity array are spaced apart from each other by a distance, said distance being chosen such that a particular resolution is achieved, a distance of xe2x80x9cdxe2x80x9d between electrodes achieving one resolution, the distance xe2x80x9c3dxe2x80x9d achieving still another resolution, and the distances xe2x80x9c6dxe2x80x9d and xe2x80x9c9dxe2x80x9d achieving still another resolution. The larger the distance between the electrodes, the deeper into the earth formation the electrical current will flow, and the deeper the depth of investigation on sensitivity. Electrodes spaced at distances xe2x80x9c6dxe2x80x9d and xe2x80x9c9dxe2x80x9d are sensitive to a seawater/freshwater boundary situated away from the well containing the permanently installed electrode array. This allows the presence of seawater to be determined before it arrives in physical contact with the monitoring electrode array.
It is a further aspect of the present invention to install a resistivity array around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, wherein each resistivity subarray could comprise a distinct unit comprised of an insulating material (such as plastic or ceramic) having interleaved integral electrodes, or the resistivity subarray could comprise a set of solid plated metal electrodes wrapped around a cable.
It is a further aspect of the present invention to install a resistivity array around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, wherein a plurality of isolated additional electrodes are used in combination with the resistivity array in order to monitor a free water level inside the observation well.
It is a further aspect of the present invention to install a resistivity array around an observation well for the purpose of measuring for and determining the presence or absence of a seawater/fresh water boundary in a water sand unit located adjacent to the observation well, where each subarray of the resistivity array that is located adjacent the top part and the middle part and the bottom part of the water sand unit comprises a quadrapole electrode set, one quadrapole electrode set being located adjacent the top part, another quadrapole electrode set being located adjacent the middle part, and another quadrapole electrode set being located adjacent the bottom part of the water sand unit.
In accordance with the above object and aspects of the present invention, sensors are permanently placed in the ground near observation and injection wells in order to passively and continuously monitor the status of seawater advance toward fresh water aquifers near coastal cities as well as the status of fresh water injected into the injection wells. Such sensor devices are installed in the ground and electrically connected to surface acquisition equipment that would, without human intervention, transmit acquired data to a centralized facility for processing and interpretation. This would avoid the process of manual data collection and provide more frequent and timely data for better control of water injection. Various types of sensors can be used: the sensors used for general reservoir monitoring that are disclosed in the xe2x80x98first referencexe2x80x99 cited above, and/or the sensors used for leak detection, soil heating, and temperature mapping that are disclosed in the xe2x80x98second set of referencesxe2x80x99 cited above. Alternatively, a special type of sensor can be designed and provided for the purpose of monitoring the status of seawater advance toward fresh water aquifers.
An array of such sensors (hereinafter called a xe2x80x9cresistivity arrayxe2x80x9d) is installed in the Earth along the exterior of the injection or observation well casings, or such sensors can be placed inside the well. Then, time-lapse in-situ electrical resistivity measurements are carried out. A basic quadrapole measurement consists of injecting and withdrawing current (I) between two outer electrodes of a group of four electrodes, and measuring the voltage potential (V) between the two inner electrodes. The resistivity of the Earth in the vicinity of the quadrapole sensors is computed from the current xe2x80x9cIxe2x80x9d and the potential xe2x80x9cVxe2x80x9d. Thus, the electrode quadrapoles may be used to obtain resistivity measurements at one or more depths in the water bearing sands in the earth. Consequently, each single formation or water resistivity measurement may be used to discriminate between fresh water and saline (salt) water along the length of the (injection or observation) well.
Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description.